The present invention broadly concerns devices and systems for use in the fixation of the spine and correction of spinal disorders. In one aspect, the invention concerns a bone engaging element for use in a spinal implant construct utilizing elongated fixation elements, such as spinal rods, contoured for connection at various locations along the length of the spinal column. More specifically, the invention concerns a screw for anchoring the spinal rods of the construct to the iliac wings or the pelvis.
Several techniques and systems have been developed for use in correcting spinal injuries and deformities, stabilizing spinal curves and facilitating spinal fusion. Frequently, an elongated spinal member, such as a bendable rod, is longitudinally disposed adjacent the vertebral column and is fixed to various vertebrae along the length of the column. This fixation is achieved by a number of fixation elements, which can be hooks or bone screws of various configurations.
An example of one such system is the TSRH.RTM. spinal system of Danek Medical, Inc. In one construct using the TSRH.RTM. system, shown in FIG. 1, a fixation construct 10 includes two elongated rods 11 disposed adjacent the spine on opposite sides of the spinous process. In the illustrated construct, the rod is shown extending from the thoracic vertebrae to the sacrum and pelvis. It is of course known that fixation along the entire length of the spine up to the cervical vertebrae can also be accomplished using a rod system such as the TSRH.RTM. system.
In this system 10, a variety of bone engaging fasteners 15 are engaged to the rods 11 by way of eyebolt assemblies 16. These eyebolt assemblies are known components of the TSRH.RTM. system and are particularly distinguished for the "three-point shear clamp" effect achieved by the eyebolt assemblies to clamp the bone engaging fasteners 15 to the spinal rods 11.
In many spinal instrumentations, the inferior ends 11a of the spinal rods 11 are anchored to either the sacrum or the pelvis. In FIG. 1, the spinal fixation system 10 is anchored to the iliac wings I according to the Galveston fixation technique. In accordance with this technique, a Galveston rod segment 12 is linked to the ends 11a of the spinal rods 11. The Galveston rod segments 12 each include an iliac extension 13 which is engaged within holes bored through the iliac wings I. A transverse plate 17, preferably the Crosslink.RTM. plate provided by Danek Medical, Inc., is engaged between the two Galveston rod segments 12 by way of separate eyebolt assemblies 18.
This particular Galveston fixation technique permits correction of pelvic obliquities. Perhaps more importantly, the Galveston approach provides a solid anchor for the superior fixation construct. The iliac wings have large bony regions for significant purchase of the Galveston rod segments 12. These rods and specifically the iliac extensions 13, can have a length of 85-100 mm to provide a substantial moment arm to react the large loads generated by the entire construct. The Galveston rods provide a very stable foundation for fusion at superior vertebral levels.
In the construct depicted in FIG. 1, the Galveston rod segments 12 are engaged to the spinal rods 11 by way of a pair of offset plates 20. These offset plates 20 are engaged to the respective rods by way of eyebolt assemblies 21. This offset plate 20 allows separate rods to be linked axially. This particular configuration has replaced the more complex technique in which the spinal rod itself was bent at its inferior end to engage the iliac wings I. In other words, this technique allows the substitution of a much shorter rod segment 12 already carrying the Galveston bend 14, thereby permitting ready engagement between the iliac wings and the spinal rods 11. The addition of the Crosslink.RTM. transverse plate 17 adds greater strength and rigidity to the construct and prevents pullout of the iliac extensions 13 from the iliac wings I.
Although the use of the offset plates 20 has greatly simplified anchoring to the iliac bone from the prior Galveston technique, there still is room for improvement. In particular, the shorter rod segments 12 depicted in FIG. 1 still require the Galveston bend 14 for the iliac extensions 13. It is a known principal of mechanical engineering that bends in the rods yield stress concentrations and can also produce asymmetric loading of the fixation construct. It is desirable to engage the bones of the sacrum or ilium without the requirement for bending the rods. In order to achieve this capability, a novel spinal fixation system 25 has been developed, as illustrated in FIG. 2. In this fixation construct 25, the spinal rods 11 and bone engaging fasteners 15 are identical to the similar components in the construct 10 shown in FIG. 1. The difference is that the Galveston rod segments 12 and offset plates 20 have been replaced by extension rod assemblies 27. The extension rods assemblies 27 include an extension rod 28 having a rod portion 29 and a head portion 30. The extension rods 28 are engaged to the inferior ends 11a of the spinal rods 11 by way of clamping members 31. The details of this variable angle extension rod assembly 27 are set forth in a co-pending commonly-owned patent application entitled "VARIABLE ANGLE EXTENSION ROD" filed in the name of inventors Sherman and Tai on Sep. 13, 1995. The disclosure of this application as it pertains to the details of the extension rod assembly 27 are incorporated herein by reference.
As discussed more fully in that co-pending application, the extension rod assembly 27 eliminates the need for the Galveston bends and instead provides two angular and one translational degrees of freedom to interconnect the spinal rods 11 to bone screws, such as bone screw 33 engaged in the iliac wings I.
In the construct 25 depicted in FIG. 2, the screws 33 are engaged to the extension rods 28 by way of clamping assemblies 34. As thus far described, the construct 25 of FIG. 2 can utilize a wide range of screws for engaging the iliac wings I. One option is to use the variable angle screw disclosed in U.S. Pat. No. 5,261,909 in the name of inventors Sutterlin and Sherman. This '909 patent, which is assigned to the owner of the present invention, discloses a variable angle clamping mechanism for engaging a bone screw to an elongated rod at variable angular positions in two degrees of freedom. Specifically, the variable angle clamping mechanism can be rotated around the longitudinal axis of the elongated rod. In addition, the head of the bone screw can be rotated relative to the clamping mechanism to achieve a plurality of different angular positions.
While this variable angle screw provides adequate fixation into the iliac bone, its insertion into the bone is rendered difficult because the bone screw is fully threaded. On the other hand, the use of a smooth Galveston rod, as is known in the art, can lead to a "windshield wiper effect" in which cyclic loads applied to the construct 25 gradually cause the Galveston rod to erode or whittle away at the bone opening in the iliac wing I. While the Galveston rod provides a very long moment arm for securing the base of the spinal construct, this same long moment arm can lead to loosening of the rod. On the other hand, while the variable angle screw described above can obtain a secure fixation resistant to pull-out from the iliac wings, it is not readily capable of achieving the long moment arms of the Galveston rod.
A need has remained for an iliac bone fixation member that combines the benefits of the threaded variable angle screw with the benefits of the smooth Galveston rod without the corresponding disadvantages.